The present invention relates to a method to generate a pseudo-random sequence of multi-carrier data symbols by producing a pseudo-random bit sequence by repetitively generating a pseudo-random sequence of L bits, L being a first integer value and packetizing into multi-carrier data symbols thereby using N bits of the pseudo-random bit sequence per multi-carrier data symbol, N being a second integer number, to thereby generate the pseudo-random sequence of multi-carrier data symbols, a generator of a pseudo-random sequence of multi-carrier data symbols comprising scrambling means, adapted to repetitively generate a pseudo-random sequence of L bits, L being a first integer value, to thereby produce a pseudo-random bit sequence and packetizing means, adapted to packetize into multi-carrier data symbols using N bits of said pseudo-random bit sequence per multi-carrier data symbol, N being a second integer number, to thereby generate the pseudo-random sequence of multi-carrier data symbols, a multi-carrier transmitter including a pseudo-random sequence generator and transmitting means coupled to the pseudo-random sequence generator, and adapted to transmit a pseudo-random sequence of multi-carrier symbols generated by the pseudo-random sequence generator over a communication channel, and a multi-carrier receiver including a pseudo-random sequence generator and receiving means adapted to receive a first pseudo-random sequence of multi-carrier symbols transmitted over a communication channel, and decoding means, coupled to said receiving means and to the pseudo-random sequence generator, and adapted to decode the first pseudo-random sequence of multi-carrier symbols and a second pseudo-random sequence of multi-carrier symbols generated by the pseudo-random sequence generator.
Methods and equipment to generate a pseudo-random sequence of multi-carrier data symbols are already known in the art, e.g., from the ADSL Standard Specification ‘Network and Customer Installation Interfaces—Asymmetric Digital Subscriber Line (ADSL) Metallic Interface’, published by the American National Standards Institute (ANSI) in 1998 and referred to by ANSI T1E1.413 Issue 2. According to this Standard Specification, the ADSL line termination at the central office generates a pseudo-random sequence of 16,384 DMT (Discrete Multi Tone) data symbols, each comprising 512 bits. The pseudo-random sequence of DMT data symbols, named C-MEDLEY in paragraph 9.6.6 of the just cited Standard Specification, is derived from a pseudo-random sequence of 511 bits generated repetitively by a scrambler in the ADSL line termination. The pseudo-random sequence of DMT data symbols, C-MEDLEY, is sent over a twisted pair telephone line towards the ADSL network termination at the customer premises and is used therein for downstream channel analysis. In a similar way, the ADSL network termination at the customer premises produces a pseudo-random sequence of 16,384 DMT data symbols each having a length of 64 bits, named R-MEDLEY in paragraph 9.7.8 of the above cited Standard Specification, and sends this pseudo-random sequence of DMT data symbols over the twisted pair telephone line towards the ADSL line termination at the central office for upstream channel analysis. The pseudo-random sequence of DMT data symbols, R-MEDLEY, is derived from a 63-bit long pseudo-random sequence of bits that is repetitively generated by a scrambler in the ADSL network termination.
In applications such as VDSL (Very High Speed Digital Subscriber Line), wherein the number of bits per multi-carrier data symbol, that will be named N throughout the remainder of this patent application, may have different values, two problems can occur in case the known technique is applied: the randomness of the sequence of multi-carrier data symbols may decrease significantly and/or the length of the pseudo-random sequence of multi-carrier data symbols may become short in comparison with the longest achievable pseudo-random sequence that contains L multi-carrier data symbols, L being the number of bits in the repetitively generated pseudo-random sequence of bits generated by the scrambler. Indeed, the length of the pseudo-random sequence of multi-carrier data symbols becomes short in case this number of bits per multi-carrier data symbol, N, relates in a certain way to the number of bits, L, in the pseudo-random sequence of bits that is repetitively generated by the scrambler, e.g., a.N=b.L with a and b being integer values respectively smaller than L and N. The number of bits in the repetitively generated pseudo-random sequence of bits, L, is typically equal to 2S−1 if the scrambler is implemented by a finite state machine and S represents the number of states of this finite state machine. Suppose, for instance, that S equals 9 and consequently that L equals 29−1=511. If each multi-carrier data symbol has a length N of 1022 bits, then, because N=2.L, each multi-carrier data symbol in the pseudo-random sequence of multi-carrier data symbols will consist of exactly the same pseudo-random sequence of 1022 bits. In this situation, the length of the pseudo-random sequence is only 1 multi-carrier data symbols, which means that there is in fact no randomness. In case other relations are satisfied between N and L, e.g., N=L−1 or N=L+1, some randomness is lost. This is so because the bits in the pseudo-random sequence are typically used pairwise to apply random rotations to the different carriers. In the case of N=L−1 or N=L+1, the pair of bits that defines the random rotation that will be applied to a single carrier can only differ in one bit between two successive multi-carrier data symbols, thus reducing the randomness of the rotations that are applied. When applied in a VDSL system, the known method may thus generate a pseudo-random sequence of multi-carrier data symbols with decreased randomness or which is rather short whereas a long random sequence of multi-carrier data symbols is required in order to be able to analyze the channel—in ADSL channel analysis involves SNR (Signal to Noise Ratio) estimation—accurately.